1. Field of the Invention
The invention relates to an audio output device used in apparatuses to output sounds, such as TV sets, radio-cassette players, car audios, home theaters, and component stereo sets.
2. Description of the Related Art
With recent development of LSI technologies, digital audio apparatuses, such as CD players and MD players, come to use 1-bit digital analog converters (DACs) to process and amplify digital signals. The 1-bit DAC performs noise shaping on audio signals by using a ΔΣ modulator, and then outputs the resultant audio signals as pulse-width modulated signals, specifically, as 1-bit PWM signals.
A conventional first-order ΔΣ modulator as one type of above-described ΔΣ modulator has a configuration shown in FIG. 4. FIG. 4 illustrates components of the first-order ΔΣ modulator by using a Z function obtained through the Z-transform. Note that Z−1 represents a delay element to delay the input by one sampling clock.
The first-order ΔΣ modulator shown in FIG. 4 includes: a subtractor 81; an accumulator 90 that includes an adder 82 and a delayer 84; a delayer 85; a multiplier 86; and a quantizer (Q) 83. The subtractor 81 subtracts a feedback signal W from an input signal X. The accumulator 90 accumulates outputs S of the subtractor 81 for every sampling clock.
The quantizer 83 generates binary-quantized output signals Q(Z) by outputting “+Δ” if the output Y of the accumulator 90 is equal to or larger than zero (i.e., Y≧0), and by outputting “−Δ” if the output Y of the accumulator 90 is smaller than zero (i.e., Y<0). Each output signal Q(Z) is delayed by the delayer 85 by one sampling clock, and is inputted as the feedback signal W to the subtractor 81 through the multiplier 86.
The first-order ΔΣ modulator is a feedback system with the above-described components. The first-order ΔΣ modulator is a modulator to convert the input signals X with a dynamic range from −Δ to +Δ into binary-quantized signals (+Δ, −ΔA) in synchronization with the sampling clocks for the two delayers 84 and 85.
Apparatuses to output sounds, such as TV sets and audio players by use of such ΔΣ modulators as one described above usually use higher-order ΔΣ modulators to improve the quality of the sounds. As described in Japanese Patent Application Publications Nos. Hei 9-307447, 2001-237707, and 2003-298425, a higher-order ΔΣ modulator includes plural accumulators and quantizers cascaded within a single loop.